Microporous polymeric films having structures that enable vapors to flow through them such that they are breathable or have breathability while at the same time inhibiting or stopping the flow of liquids through them have been known for some time. These types of films have been utilized in a wide variety of applications, such as in the preparation of synthetic leather, in the preparation of cloth laminates for use as synthetic shoes, raincoats, outer wear, camping equipment such as tents, and the like, and in combination with fabrics and other materials for preparation of medical applications such as surgical gowns, bandages, and the like, and applications such as house wrap, covers for automobiles and other motorized vehicles, and the like.
A variety of porous stretched polymeric films are known in the art. In general, three types of processes are typically used to manufacture porous, melt-processed polymeric films that include an orientation step. These three types are processes for stretching films of neat, unblended polymers containing no fillers other than typical stabilizing additives; processes for making films from blends of two or more polymers, or from blends of polymer with mineral oil or an organic salt dispersed in the blends in which the dispersed phase is extracted with the film stretched before or after extraction; and processes for casting films from blends of polymer with a filler such as calcium carbonate or barium sulfate with no extraction and the film oriented after casting.
Patents disclosing porous polymeric films known in the art are described in commonly assigned U.S. Pat. No. 4,975,469 which discloses oriented porous polymeric films formed from a polypropylene-based resin and having a moisture vapor transmission rate (MVTR) determined according to ASTM E-96, procedure E, in the range of about 2,500 to about 7,500 g/m.sup.2 /24 hr, and a process for forming such films. The aesthetic characteristics of these polypropylene porous films show a somewhat stiffer nature or harsher hand relative to porous film made from synthetic materials other than polypropylene and that they have a tendency to produce a "rustling" noise when flexed.
Commonly assigned U.S. Pat. No. 4,386,129 discloses a porous polymeric film having pores with polygonal cross sections and average pore diameters of about 3 to about 100 microns formed from a resinous polymer of propylene and a process for forming such film comprising the steps of forming film containing beta-spherulites and subsequently selectively extracting the beta-spherulites. Such films have utility as filtration devices, raincoats and tents.
U.S. Pat. No. 5,008,296 discloses microporous films prepared by casting film from compositions of a polyolefin and high levels of CaCO.sub.3 or glass beads and calcium stearate and biaxially stretching the film from 1.5 to 7 times in each direction with the resulting film having Gurley porosities of from 0.1 to 85 seconds.
Despite the various porous films and methods of preparing same disclosed in the patents discussed above, there remains a need for polymeric compositions capable of being formed into microporous films having increased elongation, improved breathability and for a facile process for forming such microporous films.
It is an object of this invention to provide improved polymeric compositions capable of being converted into oriented microporous polymeric films. Another object of this invention is to provide oriented microporous polymeric film having improved elongation, tear strength, softness and lower "noise level" upon handling or flexing. Other objects of this invention will be apparent to persons skilled in the art from the following description and claims.
Advantageously, we have found that the polymeric compositions of this invention provide oriented microporous films that exhibit improved extensibility and that are useful for apparel applications such as sportswear, ski clothing, lining material for such apparel, medical apparel such as surgical drapes, protective gowns and garments, equipment covers, filters, housewrap, pressure sensitive labels and the like.